A conventional in-wheel motor driving device 101 is disclosed in JP-A-2009-52630 (Patent Literature 1) for example.
As shown in FIG. 16, the in-wheel motor driving device 101 includes a housing 102 which is attached to a vehicle body; a motor section 103 which is placed therein and generates a driving force; a wheel hub bearing section 104 which is connected to a wheel; and a speed reducer section 105 which reduces rotating speed of the motor section 103 and transmits the rotation to the wheel hub bearing section 104.
In the in-wheel motor driving device 101 of the above-described construction, a low-torque high-rotation motor is utilized for the motor section 103 in view of reducing the size of the device. On the other hand, the wheel hub bearing section 104 requires a large torque in order to drive the wheel. For these reasons, a cycloid reduction gear system is often utilized for the speed reducer section 105 due to its compactness and high speed-reduction ratio.
A speed reducer section 105 utilizing a cycloid reduction gear system includes a motor-side rotation member 106 which has eccentric sections 106a, 106b; cycloid discs 107a, 107b which are disposed in the eccentric sections 106a, 106b; roller bearings 106c which rotatably support the cycloid discs 107a, 107b with respect to the motor-side rotation member 106; a plurality of outer circumferential engagers 108 which make engagement with outer circumferential surfaces of the cycloid discs 107a, 107b to generate rotational movement of the cycloid discs 107a, 107b; and a plurality of inner pins 109 which transmit the rotational movement of the cycloid discs 107a, 107b to the wheel-side rotation member 110.
The outer circumferential engager 108 is not held directly by the housing 102a of the speed reducer section 105, but is held by an outer circumferential engager holding section 113 which is provided on an inner diameter surface of the housing 102a. More specifically, it is held rotatably by needle bearings 114 which have their axial end portions fixed to the outer circumferential engager holding section 113. By making the outer circumferential engager 108 rotatable with respect to the outer circumferential engager holding section 113 in this way, contact resistance caused by engagement with the cycloid discs 107a, 107b is reduced.